The present invention relates to a preparatory station, in particular for a lithographic projection apparatus.
An apparatus of this type can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can then be imaged onto a target area (die) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent dies that are successively irradiated through the reticle, one at a time. In one type of lithographic projection apparatus, each die is irradiated by exposing the entire reticle pattern onto the die at once; such an apparatus is commonly referred to as a waferstepper. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatusxe2x80x94each die is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the wafer parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed v at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205, for example.
Up to very recently, apparatus of this type contained a single mask table and a single substrate table. However, machines are now becoming available in which there are at least two independently movable substrate tables; see, for example, the multi-stage apparatus described in International Patent Applications WO 98/28665 and WO 98/40791. The basic operating principle behind such multi-stage apparatus is that, while a first substrate table is underneath the projection system so as to allow exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge an exposed substrate, pick up a new substrate, perform some initial measurements on the new substrate, and then stand by to transfer this new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed, whence the cycle repeats itself; in this manner, it is possible to achieve a substantially increased machine throughput, which in turn improves the cost of ownership of the machine.
Lithographic apparatus may employ various types of projection radiation, such as ultra-violet light (UV), extreme UV, X-rays, ion beams or electron beams, for example. Depending on the type of radiation used and the particular design requirements of the apparatus, the projection system may be refractive, reflective or catadioptric, for example, and may comprise vitreous components, grazing-incidence mirrors, selective multi-layer coatings, magnetic and/or electrostatic field lenses, etc; for simplicity, such components may be loosely referred to in this text, either singly or collectively, as a xe2x80x9clensxe2x80x9d. The apparatus may comprise components that are operated in vacuum, and are correspondingly vacuum-compatible. As mentioned in the previous paragraph, the apparatus may have more than one substrate table and/or mask table.
In a manufacturing process using a lithographic projection apparatus, a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy sensitive material (resist). For this process it is necessary to provide the substrate to the substrate table and to hold the substrate firmly in a fixed position on said table during the process. The substrate can be held in this fixed position with a substrate holder comprising means for applying a vacuum to a major surface of the substrate table. The vacuum will suck the substrate firmly to the substrate table. Before the substrate is provided to the substrate table it will often be processed (e.g. spincoated with resist) in a process track and therefore the temperature of the substrate can be different to the temperature of the substrate table. This temperature difference can give a problem, because the temperature of the substrate can change after the substrate is provided to the substrate table. The substrate table can cool the substrate, so that the substrate would tend to shrink. However, the substrate is held firmly by the substrate holder, which does not allow the substrate to shrink. The substrate can only shrink when the tension inside the substrate is higher than the friction between the substrate and the surface of the substrate table. If this occurs, a part of the substrate will slip over the surface of the substrate table to release the tension inside the substrate. This slip movement can give an error in the super-positioning of two concurrent images exposed on successive layers on the substrate, leading to so-called overlay errors. In general, the super-positioning of two concurrent images is very accurately achieved by aligning a mark on the substrate to a reference mark (e.g. on the mask, or on a fiducial on the substrate table). If the substrate slips after aligning the substrate to said reference mark, the super-positioning of two concurrent images on the slipped part of the substrate can fail. Similar considerations apply to the case whereby the substrate is colder than the substrate table and is warmed by the substrate table. The substrate in that case would like to expand and can also slip over the surface of the substrate table.
It is an object of the invention to alleviate, at least partially, the above problems. Accordingly the present invention provides an apparatus having an intermediate table comprising a major surface provided with a plurality of apertures, and gas bearing means for generating a gas bearing between said major surface and a substrate located thereon.
The gas bearing substantially removes the friction between the substrate and the major surface of the intermediate table. The substrate can easily expand and shrink on the gas bearing when the temperature of the substrate changes. Another advantage of using a gas bearing between the surface of the intermediate table and the substrate is that contamination of the backside of the substrate by foreign particles present on the major surface of the intermediate table is avoided. Particles already collected on the backside can even be blown away from the backside by the gas bearing.
The gas used in the gas bearing can be air and the gas source can be provided with a filter for the gas (e.g. air from outside) such that it is substantially free of foreign particles. Alternatively, other gases can be used, for example, nitrogen or helium. As will be appreciated by the skilled artisan, one can control the gas bearing by having apertures for the inflow of gas to the gas bearing and also apertures for the evacuation of gas from the gas bearing. A particular pressure for the inflow of gas may be between about 1.1 and 1.5 bar whereas a reduced gas pressure for the evacuation of the gas may be between about 0.5 and the 0.9 bar, for example. The gas bearing may have a thickness less than about 150 xcexcm, for example.
The preparatory station can comprise a gas ionizer to ionize the gas used to create the gas bearing By using the gas ionizer the substrate can be gradually discharged from any initially charged state (since a statically charged part of the substrate will attract ions with an opposite charge, so that the charged part will be neutralized by the ions). This gradual discharging is advantageous, because it prevents a sudden discharge of the substrate, e.g. if it comes into the neighborhood of a conductor. A sudden discharge, for example with a spark, can cause damage to the substrate or to the sensitive structures already created thereon. As will be appreciated by one of ordinary skill in the art, the gas ionizer can, for example, employ radioactive ionization or corona discharge to ionize the gas; corona discharge in a method applying a high voltage to a sharp point to ionize the gas in the vicinity of the point.
The intermediate table can comprise a first controller to control the temperature of that table. By controlling temperature of the intermediate table, the temperature of the substrate can be influenced. A first possible mechanism for effecting such influence can be thermal radiation from the substrate to the intermediate table. A second mechanism can be that the temperature of the intermediate table influences the temperature of the gas used in the gas bearing, and the temperature of the gas influences the temperature of the substrate. Especially when the gap caused by the gas bearing between the substrate and the surface of the intermediate table is very thin, the temperature of said table can have a strong and rapid influence on the temperature of the substrate.
The preparatory station can comprise a second controller to regulate the temperature of the gas used in the gas bearing. By directly regulating the temperature of said gas, the temperature of the substrate can be influenced as well. Especially when the gap between the substrate and the intermediate table is large, it may be desirable to regulate the temperature of the gas directly instead of by regulating only the temperature of the intermediate table.
An advantage of the invention as described above is that said first and second controllers can maintain the intermediate table and the gas at a temperature substantially equal to the temperature of the substrate table (e.g. as measured using temperature sensing means in the substrate table). In that case the temperature of the gas and the intermediate table will change the temperature of the substrate to a temperature substantially equal to the temperature of the substrate table. After the substrate is provided to the substrate table the temperature of the substrate will not change significantly anymore, and therefore no substantial expansion or shrinkage of the substrate will occur on the substrate table. Accordingly, the chance that a slip of the substrate on the substrate table will occur can be minimized when these measures are taken.
In another embodiment according to the invention, the said preparatory station further comprises:
a detectors constructed and arranged to detect a first position of said substrate on said intermediate table;
a calculator constructed and arranged to calculate a required displacement between said first position and a desired position of the substrate on the intermediate table; and
an actuator constructed and arrange to move said substrate from said first position to said desired position.
The detector can comprise an edge detector for detecting the position of the edge of said substrate on said intermediate table. The detector can comprise capacitive sensors or optical sensors, e.g. a camera system or a CCD array. The information obtained with the detector about the first position of the substrate on the intermediate table, together with information regarding the desired position of the object on the table, can be processed in the calculator so as to calculate the required displacement. Said desired position of the substrate on the intermediate table can be determined beforehand and stored in a memory device, wherefrom it can be retrieved when necessary. The actuator can comprise, for example, a robot arm.
The substrate can be brought into the desired position and to a certain temperature simultaneously. This improves the time efficiency of the apparatus (throughput-enhancement). Another advantage is that the accuracy with which the detector can detect the first position of the substrate on the intermediate table is better, because the substrate is perfectly flat on the gas bearing. This is because the gas bearing exerts a force on the substrate that is equally divided over the total backside of the substrate, so that the substrates is not stressed (and thus bent or warped) by the substrate holder.
The invention also relates to a device manufacturing method comprising:
providing substrate to an intermediate table comprising a major surface provided with a plurality of apertures, and maintaining the substrate for a given time interval upon a gas bearing generated between the major surface and the substrate. Detaining the substrate on the gas bearing in this manner allows the substrate to shrink or expand when its temperature changes without tension arising between the substrate and the intermediate table.
In a manufacturing process using a lithographic projection apparatus according to the invention, a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an-array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
The invention as explained hereabove may also be applied to a preparatory station for preparing a mask.
Although specific reference has been made to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget areaxe2x80x9d, respectively.